Expression of extended polyglutamine sequentially activates initiator and effector caspases

To date, eight neurodegenerative disorders, including Huntington's disease and dentatorubral-pallidoluysian atrophy, have been identified to be caused by expansion of a CAG repeat coding for a polyglutamine (polyQ) stretch. I t is, however, unclear how polyQ expansion mediates neuronal cell death obs erved in these disorders, Here, we have established a tetracycline-regulate d expression system producing 19 and 56 repeats of glutamine fused with gre en fluorescent protein. Induced expression of the 56 polyQ, but not of the 19 polyQ stretch caused marked nuclear aggregation and apoptotic morphologi cal changes of the nucleus. In vitro enzyme assays and Western blotting sho wed that polyQ,, expression sequentially activated initiator and effector c aspases, such as caspase-8 or -9, and caspase-3, respectively. Furthermore, using cell-permeable fluorogenic substrate, the activation of caspase-3-li ke proteases was demonstrated in intact cells with aggregated polyQ. This i s the first direct evidence that the expression of extended polyQ activates caspases and together with the previous findings that some of the products of genes responsible for CAG repeat diseases are substrates of caspase-3 i ndicates an important role of caspases in the pathogenesis of these disease s. (C) 1999 Academic Press.